The 12 people who walked on the Moon are the only humans who ever set foot on another world1. Though this world is on average more than 380.000 km away from ours, the rocks brought back by the astronauts revealed an interesting fact: While the astronauts literally travelled very far away from home, they figuratively slammed their flag in yet another piece of the Earth.

Moon rocks are in many respects similar to Earth rocks. The only explanation we have so far which sufficiently explains this similarities is the Giant Impact Theory2: A mars-sized planet (named Theia) collides with the Proto-Earth, and the Moon subsequently forms from the material that is ejected into Earth`s orbit by the collision. In this scenario the Moon is formed from mainly Proto-Earth material. Welcome home guys3.

We know a lot about Earth`s surface. After all, we can easily walk around and pick up rocks nearly everywhere4. And even better, we also know quite some stuff about its interior through the information we get through earthquakes and rocks brought up from the deep through volcanism. We know the Earth has an iron rich core (divided into a solid inner and liquid outer core), a stiff inner mantle and a rigid outer mantle surrounded by a thin crust (Figure 1). We have a similar yet much less detailed picture of the lunar interior (Figure 2). And we think that all other rocky planets (Mercury, Venus, Mars, Pluto), dwarf planets (Pluto, Ceres …), and for that matter exoplanets of similar composition and size have similar structures. The main principle causing these structures is that heavy elements (such as iron and nickel) sink to the centre of a planet to form the core while lighter elements are preferentially found in the outer part of the planet.

Figure 2: The Moon`s interior. Yes, there were even some moonquakes measured by the equipment set up by the Apollo missions. From Wieczorek et al. (2006).

But we have to be carefully to not overstress when we apply our knowledge of the Earth to similar bodies. A dominant mineral in the Earth`s upper mantle is olivine. So it`s fair to assume for a start that it is also abundant in the Moon`s upper mantle5 (Figure 3). This olivine would be excavated by big impact events on the Moon if they penetrate deep enough. A study (Melosh et al., 2014) presented at this year’s Lunar and Planetary Science Conference showed by modelling that the South Pole-Aitken (SPA) impact event most certainly excavated mantle material. Therefor there should be a substantial amount of olivine in the area around the impact basin where the ejected material from the upper mantle came to rest. Luckily6 olivine is one of the minerals that can be detected by NASA`s Moon Mineralogy Mapper (M3 or M-cubed). So the scientist used the data set provided by M3 to have a look on a region that should be dominated by SPA ejecta. What they found was nearly no trace of olivine but a dominant signature of another mineral also found in the Earth`s mantle – orthopyroxene. This indicates that the lunar upper mantle might be dominated by orthopyroxene in contrast to the Earth`s olivine dominated upper mantle.7

Figure 3: A possible model for minerals in the Moon`s interior. From Wieczorek et al. (2006).

If this hypothesis turns out to be correct, how could this be explained, taking into account that Earth and Moon started with a similar chemical composition after the Giant Impact?

The experts on that matter will probably find a detailed explanation in due time. I`m not an expert on this, so please consider the next paragraph wild speculation:

Two main factors that control which minerals are formed from a given chemical composition are temperature and pressure. And they might have been much different in the lunar upper mantle at the time it crystallized compared to Earth. This is due to the mass difference between the Earth and the Moon8. This means the pressures that can be reached within the Moon are lower than on Earth. Also the lower mass affected the thermal history of the Moon so that it cooled down much faster than the Earth. Therefor this two key factors for the formation of minerals were probably different at the time of crystallization of the lunar upper mantle compared to Earth`s upper mantle.

These key factors will also be different in other planetary bodies and therefore we might see similar effects there too. Melosh et al. (2014) themselves refer to the asteroid “4 Vesta” on which similar studies revealed substantial amounts of orthopyroxene in material believed to be from the upper mantle as well (McSween et al., 2013).

Libourel and Corrigan (2014) generally point out that there is a scarcity of olivine dominated material in asteroid observations: Scarce if you assume that asteroids differentiate into an iron-nickel core and an olivine-dominated mantle. This of course is the same (Earth-centered) assumption that brought us into trouble with the lunar mantle as well.

1) While the 6 command module pilots were probably the humans who were farthest away from any other human being – ever.

2) There are some problems with this theory. Therefor its details are under revision at the moment. I hope I`ll be able to write something up on that soon. So stay tuned.

3) Man, that`s a bit like traveling over a dangerous ocean to stick your flag into a New World just to discover some (distant) cousins of yours are already there. Uhm, there seems to be a pattern here …

4) Well, on the continents that is. And if the region is not too dangerous. And if you have enough founding to go there.

5) Especially as there is the possibility that a lot of olivine from the lower mantle might have been ended up in the upper mantle through a massive mantle overturn. But that`s another story.

7) I`m almost certain that I read a recent paper before going to LPSC that similarly suggested that something funny was going on with the lunar mantle (and maybe even mentioning that there was less olivine than expected). I thought it was in Spudis et al. (2014) but going back to that paper I can`t find any mentioning of that matter. So maybe I just mixed that up as I was reading the Melosh et al. (2014) abstract and the Spudis et al. (2014) paper around the same time, or there was another paper. So if anyone out there remembers reading something on that matter, please let me know.